The monitoring of particle size distribution (PSD) in media such as suspensions is important in many industrial and diagnostic applications. Control or monitoring of operations such as crystallization, filtration, combustion, mineral processing, preparation of reaction feed streams, degree of reaction and the like, often depend on the ability to monitor and control the size of particles that are components of gas or liquid process streams.
Crystallizers are widely used in industrial processes for the production of chemicals, food products and pharmaceuticals. Crystallization processes are dynamic in nature and variations in crystal (particle) content and size are expected in real-time. Monitoring of these changes in particle content and size is essential to obtain/maintain desirable operating conditions, avoid complications in down stream processing and ensure product quality/conformity.
A number of methods of monitoring PSD are known in the art. The most widely accepted standard for PSD measurements is based on laser diffraction which, however, is restricted to diluted suspensions. A prior art laser-based method capable of operating in dense suspensions is the Focused Beam Reflectance Method (FBRM). However, this method measures the chord length distribution and not the particle size distribution. This method is also prone to errors caused by particle shadowing (due to fine particles), particle masking (due to coarse particles), and assumes that the entire particle projection area has perfect back reflectance. Furthermore, measurement samples are localized and not representative of the bulk and medium transparency is essential.
Ultrasonic attenuation spectroscopy has widely been accepted as one of the most promising techniques for measuring PSD in dense and opaque suspensions. However, its applicability for online measurement in dense suspensions generally has been restricted to smaller particles (colloids and emulsions) primarily due to the unavailability of a theoretical model for larger particle sizes.
A model for predicting PSD requires accurate measurement of an attenuation spectrum at different frequencies. Recent advances in hardware and measurements for ultrasound generation have provided the ability to accurately measure the attenuation spectrum over a wide range of frequencies. The measured attenuation spectrum is then compared with the predictions of a theoretical model which requires the physical properties of the particles and suspension medium along with an assumed size distribution. A deconvolution algorithm optimizes the parameters of the assumed size distribution to minimize the error between the measured and predicted attenuation spectrum. However, the accuracy of the predicted PSD is limited by the accuracy of its theoretical model and the adequacy with which the deconvolution algorithm is able to simulate the actual conditions existing during the measurement of the attenuation spectrum.
In colloidal and emulsion systems the wavelength (λ) of the ultrasonic signal is much larger than the particle size (λ>>r) and absorption losses are dominant. This regime of wave propagation is known as the long wave regime and predominates where the ratio of particle circumference and wavelength is less than 0.11. This ratio is a non-dimensional quantity and is known as the wave number. Short wave regime of propagation exists when the wavelength is much smaller than the particle size (λ<<r). The regime of wave propagation between these two extreme limits is known as the intermediate regime.
U.S. Pat. No. 5,121,629 discloses a method of determining size distribution and concentration of particles in suspension using ultrasonic attenuation. A measured attenuation spectrum is obtained at selected discrete frequencies over a selected frequency range and compared to calculated attenuation spectra to derive an approximate match between the calculated and measured spectra. The particle size distribution and concentration used to calculate the spectra are used to derive a new set values for the particle size distribution that corresponds to the measured attenuation spectrum.
U.S. Pat. No. 7,010,979 discloses a particle size distribution monitor in liquid using ultrasonic attenuation, means of generating and receiving the ultrasonic wave and attenuation spectrum, calculation using FFT for particles suspended in the liquid. This is followed by a means of determining an estimated PSD, and a means of determining the goodness of fit.
Current ultrasonic attenuation based instruments (OPUSTM—Sympatec Inc., UltraPS—CSIRO) with the capability of online monitoring of PSD in dense suspensions of large particles (particles outside the long wave regime) are based on pre-measured attenuation coefficient spectrum of various particle size fractions. The deconvolution algorithm used by these products is highly iterative and can be unstable in the absence of any real theoretical relationship between attenuation coefficients, physical properties of the particles and size parameters. These instruments have to be extensively calibrated and customized for specific particulate systems, and the calibration process is specific for a given type and size range of particles.
Despite significant improvements in the methods and instruments available to measure PSD in media, there remain certain applications where such measurement is difficult. For example, in media with relatively larger particle size (intermediate wave propagation regime), in media that are optically opaque—such as crude oil—or where particle concentration is high, and where measurements are required on a rapid basis—even on a real-time basis—and without dilution of the fluid, conventional PSD systems are unsuitable. Accordingly, it would be useful to provide a method for measuring PSD in dense or optically opaque media in the intermediate wave propagation regime. It would also be useful to provide such a method that could provide such measurements on a rapid, or real-time basis and without extensive calibration. It would also be useful if such a method could perform these measurements without diluting the fluid in which the particles are carried.